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              <h1>TCP 拥塞控制机制 - 交互式测验</h1>
              <p>检验你对文档内容的理解程度</p>
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    "question": "在TCP拥塞控制中，当拥塞窗口（cwnd）的大小达到或超过慢启动门限（ssthresh）时，会发生什么状态转换？",
    "options": [
      "从拥塞避免阶段转换到慢启动阶段",
      "TCP连接会立即进入快速恢复阶段",
      "从慢启动阶段转换到拥塞避免阶段",
      "TCP连接会重置，cwnd降为1 MSS"
    ],
    "correctAnswerIndex": 2,
    "explanation": "慢启动门限（ssthresh）是区分慢启动和拥塞避免这两个阶段的关键阈值。在连接初期，TCP处于慢启动阶段，拥塞窗口（cwnd）会呈指数级快速增长。一旦cwnd的值达到或超过ssthresh，TCP就认为网络可能接近其承载极限，于是转换到拥塞避免阶段，此时cwnd的增长方式变为更平缓的线性增长，以更谨慎地探测可用带宽。",
    "source": "文档中「拥塞窗口与慢启动门限」- 3. cwnd 和 ssthresh 的关系 部分",
    "hint": "思考一下，ssthresh 这个“门限”的作用是什么？它是从哪个阶段进入哪个阶段的界线？"
  },
  {
    "question": "根据文档描述，TCP的“快速重传（Fast Retransmit）”机制通常由什么事件触发？",
    "options": [
      "发送方接收到三个或更多重复的确认（ACK）",
      "发送方设定的重传计时器超时",
      "网络的往返时延（RTT）突然大幅增加",
      "接收方窗口（rwnd）变为零"
    ],
    "correctAnswerIndex": 0,
    "explanation": "快速重传机制旨在比等待超时更快地响应丢包。当网络中一个数据包丢失，但其后的数据包成功到达接收方时，接收方会为每个后续到达的包重复发送对最后一个按序收到的包的确认（ACK）。当发送方接收到三个这样的重复ACK时，它会推断该数据包已丢失，并立即重传，而无需等待重传计时器到期。",
    "source": "文档中「TCP 拥塞控制的主要算法」- 3. 快速重传（Fast Retransmit）部分",
    "hint": "这个机制的“快速”之处在于它不依赖于“等待超时”。它依赖于接收方发送的什么信号来推断丢包？"
  },
  {
    "question": "在拥塞控制的“慢启动”和“拥塞避免”两个阶段，拥塞窗口（cwnd）的增长方式有何根本不同？",
    "options": [
      "慢启动阶段线性增长，拥塞避免阶段指数增长",
      "慢启动阶段cwnd固定不变，拥塞避免阶段线性增长",
      "慢启动阶段指数增长，拥塞避免阶段线性增长",
      "两个阶段都呈指数增长，但慢启动阶段速度更快"
    ],
    "correctAnswerIndex": 2,
    "explanation": "这两个阶段的增长模式是TCP拥塞控制的核心区别。在“慢启动”阶段，目标是快速利用网络带宽，因此cwnd每经过一个RTT就会翻倍，呈现指数级增长。当进入“拥塞避免”阶段后，为防止过度拥塞，增长变得保守，cwnd每经过一个RTT只增加一个MSS，呈现线性增长。",
    "source": "文档中「拥塞窗口与慢启动门限」- 1. 拥塞窗口（cwnd）- 变化规则 部分",
    "hint": "连接刚开始时，TCP会比较“激进”地探测带宽；当感觉快到极限时，它会变得“保守”。这两种行为分别对应哪种数学增长模式？"
  },
  {
    "question": "文档中提到的BBR（Bottleneck Bandwidth and Round-trip propagation time）算法，其拥塞控制的核心思想与传统算法（如Reno）最主要的区别是什么？",
    "options": [
      "BBR完全不使用拥塞窗口（cwnd）",
      "BBR依赖于明确的拥塞通知（ECN），而传统算法不依赖",
      "BBR通过主动测量瓶颈带宽和RTT来调整速率，而非依赖丢包作为拥塞信号",
      "BBR在检测到丢包后，会将发送速率降得比传统算法更低"
    ],
    "correctAnswerIndex": 2,
    "explanation": "传统TCP拥塞控制算法是“基于丢包”的，它们不断增加发送速率直到发生丢包，然后才降低速率。这种方法的缺点是会主动造成网络缓冲区膨胀和数据包丢失。而BBR算法是“基于模型”的，它不依赖丢包，而是通过持续测量网络的瓶颈带宽和往返传播时间（RTT），建立网络路径模型，并尝试将发送速率与估算出的网络容量相匹配，从而在高吞吐量的同时实现更低的延迟和丢包率。",
    "source": "文档中「TCP 拥塞控制的优化与扩展」- 1. BBR（Bottleneck Bandwidth and Round-trip propagation time）算法 部分",
    "hint": "传统算法是被动地“撞到墙（丢包）”才知道减速，而BBR试图主动地“测量墙有多远”，从而避免撞墙。思考一下BBR是如何测量的？"
  },
  {
    "question": "当TCP连接中发生丢包事件（例如触发了快速重传）时，慢启动门限（ssthresh）通常会如何调整？",
    "options": [
      "ssthresh会被设置为当前拥塞窗口（cwnd）的一半",
      "ssthresh保持不变，只有cwnd会减少",
      "ssthresh会被重置为一个非常大的初始值",
      "ssthresh会增加一倍，以期快速恢复"
    ],
    "correctAnswerIndex": 0,
    "explanation": "当发生丢包时，TCP认为当前的拥塞窗口（cwnd）值已经超出了网络的承载能力。因此，它会将慢启动门限（ssthresh）设置为当前cwnd值的一半。这个操作相当于“记住”了网络发生拥塞的大致点，在后续的恢复过程中，当cwnd增长到这个新的、更低的ssthresh时，就会提前从激进的慢启动阶段切换到保守的拥塞避免阶段，防止再次快速导致拥塞。",
    "source": "文档中「拥塞窗口与慢启动门限」- 3. cwnd 和 ssthresh 的关系 部分",
    "hint": "当检测到拥塞时，TCP需要更新一个“警戒线”，这个新的警戒线应该设置在哪里才合理？"
  },
  {
    "question": "根据文档，以下哪一项不属于TCP拥塞控制的核心目标？",
    "options": [
      "确保不同连接公平地共享网络带宽",
      "最大限度地利用网络带宽，提高吞吐量",
      "为实时应用保证绝对的零延迟传输",
      "避免网络过载和长时间的拥塞恢复"
    ],
    "correctAnswerIndex": 2,
    "explanation": "TCP拥塞控制旨在通过动态调节来平衡网络负载，其核心目标包括避免网络过载、保证公平性、最大化网络利用率以及快速从拥塞中恢复。然而，它无法保证绝对的零延迟。网络传输本身就有固有的传播延迟、处理延迟等，拥塞控制能做的是尽量减少因网络拥塞导致的排队延迟，但零延迟是一个物理上无法实现的目标。",
    "source": "文档中「拥塞控制的核心目标」部分",
    "hint": "思考一下网络传输的物理限制。拥塞控制是一种管理和优化机制，但它能克服物理定律吗？"
  },
  {
    "question": "“快速恢复（Fast Recovery）”算法的主要目的是什么？",
    "options": [
      "在检测到3个重复ACK后，立即将cwnd降至1并重新开始慢启动",
      "在发生丢包后，允许网络吞吐量更快地恢复，而不是完全重置到慢启动状态",
      "通过周期性地降低发送速率来主动避免任何可能的拥塞",
      "仅在发生超时重传后才启动，用于加速连接重建"
    ],
    "correctAnswerIndex": 1,
    "explanation": "快速恢复通常与快速重传一同工作。当快速重传被触发时，表明网络可能只是发生了轻微的拥塞（单个丢包），而不是网络崩溃。因此，完全退回到慢启动（将cwnd降至1）会过度降低性能。快速恢复算法通过将cwnd减半（而不是降为1）并直接进入拥塞避免阶段，使得TCP可以在丢包后更快地恢复其传输速率，从而提高整体的吞吐量和效率。",
    "source": "文档中「TCP 拥塞控制的主要算法」- 4. 快速恢复（Fast Recovery）部分",
    "hint": "这个算法的名称中带有“快速”二字。想一想，与彻底“从头再来”相比，什么样的方式能让恢复过程更快？"
  }
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                    child.classList.add('correct'); // 需要添加CSS样式
                    child.style.backgroundColor = '#d4edda';
                    child.style.borderColor = '#c3e6cb';
                }
                // 如果选错了，高亮错误选项
                if (!isCorrect && index === userAnswers[currentQuestionIndex]) {
                    child.classList.add('incorrect'); // 需要添加CSS样式
                    child.style.backgroundColor = '#f8d7da';
                    child.style.borderColor = '#f5c6cb';
                }
            });
        }

        function goToNextQuestion() {
            if (currentQuestionIndex < quizData.length - 1) {
                currentQuestionIndex++;
                loadQuestion();
            } else {
                showFinalScore();
            }
        }

        function goToPreviousQuestion() {
            if (currentQuestionIndex > 0) {
                currentQuestionIndex--;
                loadQuestion();
            }
        }
        
        function showHint() {
            const currentQuestion = quizData[currentQuestionIndex];
            if (currentQuestion.hint) {
                hintContainerEl.textContent = currentQuestion.hint;
                hintContainerEl.style.display = 'block';
            } else {
                hintContainerEl.style.display = 'none';
            }
        }

        function showFinalScore() {
            document.querySelector('.quiz-body').style.display = 'none';
            document.getElementById('quiz-footer').style.display = 'none';

            const correctCount = score;
            const totalQuestions = quizData.length;
            const answeredCount = userAnswers.filter(a => a !== null).length;
            const wrongCount = answeredCount - correctCount;
            const skippedCount = totalQuestions - answeredCount;
            const accuracy = totalQuestions > 0 ? Math.round((correctCount / totalQuestions) * 100) : 0;
            const errorRate = totalQuestions > 0 ? Math.round((wrongCount / totalQuestions) * 100) : 0;

            const finalScoreHtml =
                '<div class="final-score-container">' +
                '<h2>答题报告</h2>' +
                '<div class="score-summary">' +
                '<div class="score-card">' +
                '<h3>得分</h3>' +
                '<p>' + correctCount + ' / ' + totalQuestions + '</p>' +
                '</div>' +
                '<div class="score-card">' +
                '<h3>正确率</h3>' +
                '<p>' + accuracy + '%</p>' +
                '</div>' +
                '<div class="score-card">' +
                '<h3>错误率</h3>' +
                '<p>' + errorRate + '%</p>' +
                '</div>' +
                '<div class="score-card score-details">' +
                '<table>' +
                '<tr><td>正确</td><td>' + correctCount + '</td></tr>' +
                '<tr><td>错误</td><td>' + wrongCount + '</td></tr>' +
                '<tr><td>未答</td><td>' + skippedCount + '</td></tr>' +
                '</table>' +
                '</div>' +
                '</div>' +
                '<div class="report-actions">' +
                '<button id="review-btn">回顾测验</button>' +
                '<button class="primary" onclick="location.reload()">再试一次</button>' +
                '</div>' +
                '</div>';

            let reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }
            quizContentEl.insertAdjacentHTML('beforeend', finalScoreHtml);
            document.getElementById('review-btn').addEventListener('click', reviewQuiz);
        }

        function reviewQuiz() {
            const reportEl = document.querySelector('.final-score-container');
            if (reportEl) {
                reportEl.remove();
            }

            document.querySelector('.quiz-body').style.display = 'block';
            document.getElementById('quiz-footer').style.display = 'flex';
            
            currentQuestionIndex = 0;
            loadQuestion();
            updateNavigationButtons(); // Ensure nav buttons are correctly displayed
        }

        // Event Delegation for options
        optionsListEl.addEventListener('click', (event) => {
            if (event.target && event.target.matches('li.option-item')) {
                // Check if the question has already been answered
                if (userAnswers[currentQuestionIndex] === null) {
                    const selectedIndex = parseInt(event.target.dataset.index, 10);
                    selectOption(selectedIndex);
                }
            }
        });

        prevBtn.addEventListener('click', goToPreviousQuestion);
        nextBtn.addEventListener('click', goToNextQuestion);
        hintBtn.addEventListener('click', showHint);
        
        // 初始加载
        loadQuestion();
      </script>
    </body>
    </html>
  